1. Field of the Invention
The present invention relates to a solar cell and a method of making the same. More particularly, this invention relates to a solar cell with an improved rear surface structure, a solar module and a solar system composed of such solar cell.
2. Description of the Prior Art
For decades, the problems of the petroleum shortage and the greenhouse effect are getting worse and worse; therefore many countries invent more and more efforts in the development of energy saving products, such as solar cell. Worldwide solar system installations increased by 5,948 MW in 2008, up from 2,826 MW installed during the previous year. In 1985, annual solar installation demand was only 21 Megawatts. The market size of solar cell industry has increased several times from 1985 and has become a hot business coming after the semiconductor. The solar cells can be manufactured using a variety of semiconductor material or organic compound material, however, from the industrial viewpoint; silicon which is semiconductor is mainly used for the solar cells.
The solar cells using silicon can be roughly classified into a bulk type solar cell using a wafer of mono-crystalline silicon, multi-crystalline silicon or the like and a thin film solar cell having a semiconductor film formed on a substrate. The bulk type solar cell, which is the most commonly known solar cell having the largest scale of commercial market than other types of solar cells. It has been well known that in the solar cells, a good ohmic contact between the electrode and the n/p doped region will make it easier to collect the electron and hole and it will also produce a reduction of ohmic loss and thus increases the photovoltaic conversion efficiency.
In the front surface of p type solar cell, the front surface field is made by doping an n-type impurity in the whole front side. So the front side electrode can be in contact with front n doped region with good ohmic contact. In the rear surface of such solar cell, aluminum (Al), which is a cost effective material with good conductivity is most commonly used for making the p-type doped region. It has been well known that the p-type doped region can form the Back Surface Field (BSF) in the rear surface of solar cell. However, an aluminum oxide layer will be formed upon the process of forming the p-type doped region. Such aluminum oxide layer will not only increase contact resistance but also bring the difficulty for solar cell soldering.
Several technologies have been disclosed for resolving foregoing problem, for instance, by depositing a discontinued Al layer on the rear surface of solar cell to form a discontinued p-type doped region which is the most commonly used in the conventional solar cell manufacturing.
The semiconductor substrate will be exposed in such discontinued region and a silver paste, or other suitable metal pastes, can be applied to such discontinued region to form a solderable interconnection pad.
However, in such structure, the electron-hole recombination will be increased, the fill-factor (F.F.) and the short-circuit current (Isc) will be decreased. As a result, photovoltaic conversion efficiency decreases.